1. Field of the Invention
The present invention relates to a surface acoustic wave element and electronic equipment provided with the element.
2. Description of the Prior Art
A wave which is propagated by concentrating its energy in the vicinity of the surface of a propagation medium is known as a Surface Acoustic Wave (SAW).
A surface acoustic wave element is a device utilizing such a surface acoustic wave. The surface acoustic wave element is used in a band pass filter for communication equipment such as a mobile telephone, a resonator as a reference clock, a delay element for signal processing (in particular, a fourier transform functional element), various kinds of sensors such as a pressure sensor and a temperature sensor, an optical deflector, and the like.
A surface acoustic wave element used as a filter or a resonator, for example, comprises a piezoelectric base body as the propagation medium of the surface acoustic wave, a pair of interdigital transducers (ITDs) arranged on the piezoelectric base body, one for input which applies a voltage to excite a surface acoustic wave in the piezoelectric base body, and another for output which detects a surface acoustic wave propagating on the piezoelectric base body and outputs the wave by converting it into an electrical signal, and a pair of reflectors arranged on both sides of the combined IDTs. The IDTs and the reflectors are respectively formed from a layer of conductive material such as Aluminum.
In the surface acoustic wave element, when an AC power (electrical signal) is supplied to the input IDT, a strain is generated in the piezoelectric base body due to the electric field caused by the AC power. At this time, high and low density pattern is created in the piezoelectric base body because of the interdigital structure of the electrodes that generate the electric field, and as a result, surface acoustic waves are generated.
The generated surface acoustic wave propagates on both sides of the IDT, and then it is reflected by the reflectors, which gives rise to a resonance in the space between the reflectors. The energy of the surface acoustic wave is converted into electrical energy by the output IDT, and it is outputted from the output IDT.
When the surface acoustic wave element is used in a resonator or in a filter, the factors which determine the impedance characteristic of the resonator and the insertion loss in the filter include an electrode finger pitch Pt of the IDT and a reflecting body pitch Pr of the reflector.
In general, the electrode finger pitch Pt and the reflecting body pitch Pr are set respectively to about one half of the wavelength λ of the surface acoustic wave. When the thickness of the IDT and the reflector is small or the specific gravity of the constituent materials of these components is relatively low, it is possible to match the frequency of the surface acoustic wave of the IDT and the frequency of the surface acoustic wave of the reflectors by setting the electrode finger pitch Pt to be equal to the reflecting body pitch Pr, thereby enabling the impedance characteristic and the insertion loss to be improved.
However, there exist optimum values of the thickness of the IDT and the reflector in response to their required functions. For example, when a high reflectance of the surface acoustic waves of the IDT and by the reflector is desired, it is preferred that the thickness of the IDT and the reflector is relatively large.
However, when the thickness of the IDT and the reflector is made large, matching of the frequency of the surface acoustic wave of the IDT and the frequency of the surface acoustic wave of the reflector tends to be lost. As a result, a phenomenon in which the impedance characteristic and the insertion loss are likely to be impaired can be seen.
Such a phenomenon becomes particularly conspicuous when a material with relatively high specific gravity is used as the constituent material for the IDT and the reflector.
Under these circumstances, there has been proposed a surface acoustic wave element which can provide a satisfactory resonance performance by setting the electrode finger pitch Pt to be smaller than the reflecting body pitch Pr even in the case where the IDT and the reflector are configured so that their reflectance become high (see, for example, Japanese Patent Publication No. H7-73177).
However, depending upon the configuration of the piezoelectric base body, there arises a case in which no sufficient improvement in the impedance characteristic and insertion loss of the surface acoustic wave element cannot be obtained even when the configuration described in the prior art is applied.